This invention relates generally to engines of the type having combustion chambers in which an air-fuel mixture is ignited at a specific compression ratio to produce driving power. More particularly, the invention relates to an engine of this type in which the compression ratio can be variably set in accordance with the operational conditions of the engine.
It has been known that increasing the compression ratio of an engine of the general character described above results in an increase in the engine output. However, if this compression ratio is increased to a greater value than necessary, so-called knocking tends to occur at a transient time such as that during rapid acceleration, during high-speed, high load operation, or, in the case of a supercharged engine, during operation in the supercharging zone. Accordingly, in an engine of constant compression ratio, it is necessary to set this compression ratio in a restricted region in which it will not cause knocking. As a consequence, not only is the engine output at the time of normal operation subject relatively to a limitation, but the possibility of knocking becomes an obstruction to the realization of reduction in the fuel consumption.
For example, in Japanese Utility Model Laid-Open Publication No. 159746/1984 is disclosed a so-called variable compression ratio engine in which a subcompression chamber is formed to communicate with the combustion chamber of each cylinder and to slidably accommodate an adjusting piston which is actuated to advance and retract in accordance with the engine operational conditions, whereby the compression ratio, in response to an output signal of a knocking sensor, can be varied and set within a range wherein maximum torque can be obtained without occurrence of knocking.
The phenomenon of knocking is thought to be caused by a pressure vibration accompanying a rapid combustion of the end gas. Therefore, in an engine wherein, as in the above described prior art, the optimum compression ratio for each operation zone is obtained from feedback controlled by detecting the engine knocking, an abnormality in the compression pressure cannot be detected, even in the case where the combustion pressure within the combustion chamber rises suddenly, until knocking occurs. Thus, there is a limit to positively controlling the compression ratio in a zone wherein maximum torque can be obtained without occurrence of knocking.
As a measure for preventing knocking, the method of controlling the ignition timing has been widely adopted in general. However, similarly as described above, the occurance of knocking is detected by means of a knocking sensor, whereby there is a limit to bringing the ignition timing very close to the minimum ignition advance angle at which maximum torque can be obtained, and it is difficult to fully realize the potential of the engine.